The availability of safe, low-cost, long cycle life, and efficient energy storage devices is essential to increased use of renewable energy and environmentally friendly electric vehicles (EVs). Rechargeable lithium-ion (Li-ion), lithium metal, lithium-sulfur, and Li metal-air batteries are considered promising power sources for electric vehicle (EV), hybrid electric vehicle (HEV), and portable electronic devices, such as lap-top computers and mobile phones. Lithium as a metal element has the highest lithium storage capacity (3,861 mAh/g) compared to any other metal. Hence, in general, Li metal batteries (having a lithium metal anode) have a significantly higher energy density than lithium-ion batteries (having a graphite anode with a theoretical specific capacity of 372 mAh/g).
Historically, rechargeable lithium metal batteries were produced using non-lithiated compounds, such as TiS2, MoS2, MnO2, CoO2, and V2O5, as the cathode active materials, which were coupled with a lithium metal anode. When the battery was discharged, lithium ions were transferred from the lithium metal anode to the cathode through the electrolyte, and the cathode became lithiated. Unfortunately, upon repeated charges and discharges, the lithium metal resulted in the formation of dendrites at the anode that ultimately penetrated through the separator to reach the cathode, causing internal shorting, thermal runaway, and explosion. As a result of a series of accidents associated with this problem, the production of these types of secondary batteries was stopped in the early 1990's giving ways to lithium-ion batteries.
Even now, cycling stability and safety concerns remain the primary factors preventing the further commercialization of Li metal batteries (e.g. Lithium-sulfur and Lithium-transition metal oxide cells) for EV, HEV, and microelectronic device applications. Again, cycling stability and safety issues of lithium metal rechargeable batteries are primarily related to the high tendency for Li metal to form dendrite structures during cycling or overcharges, leading to internal electrical shorting and thermal runaway. This thermal runaway or even explosion is caused by the organic liquid solvents used in the electrolyte (e.g. carbonate and ether families of solvents), which are unfortunately highly volatile and flammable.
Parallel to these efforts and prompted by the aforementioned concerns over the safety of earlier lithium metal secondary batteries led to the development of lithium-ion secondary batteries, in which pure lithium metal sheet or film was replaced by carbonaceous materials (e.g. natural graphite particles) as the anode active material. The carbonaceous material absorbs lithium (through intercalation of lithium ions or atoms between graphene planes, for instance) and desorbs lithium ions during the re-charge and discharge phases, respectively, of the lithium-ion battery operation. The carbonaceous material may comprise primarily graphite that can be intercalated with lithium and the resulting graphite intercalation compound may be expressed as LixC6, where x is typically less than 1, implying a relatively low anode specific capacity (theoretically 372 mAh/g of graphite, but practically 300-360 mAh/g). Since the cathode specific capacity is typically in the range of 130-200 mAh/g, the gravimetric and volumetric energy densities of the resulting lithium-ion cell are typically in the range of 150-200 Wh/g and 450-600 Wh/L, respectively.
As a totally distinct class of energy storage device, sodium batteries have been considered an attractive alternative to lithium batteries since sodium is abundant and the production of sodium is significantly more environmentally benign compared to the production of lithium. In addition, the high cost of lithium is a major issue and Na batteries potentially can be of significantly lower cost.
There are at least two types of batteries that operate on bouncing sodium ions (Na+) back and forth between an anode and a cathode: the sodium metal battery having Na metal or alloy as the anode active material and the sodium-ion battery having a Na intercalation compound as the anode active material. Sodium ion batteries using a hard carbon-based anode active material (a Na intercalation compound) and a sodium transition metal phosphate as a cathode have been described by several research groups: X. Zhuo, et al. Journal of Power Sources 160 (2006) 698; J. Barker, et al., US Patent Application US2005/0238961, 2005; J. Barker, et al. “Sodium Ion Batteries,” U.S. Pat. No. 7,759,008 (Jul. 20, 2010 and J. F. Whitacre, et al. “Na4Mn9O18 as a positive electrode material for an aqueous electrolyte sodium-ion energy storage device,” Electrochemistry Communications 12 (2010) 463-466.
However, these sodium-based devices exhibit even lower specific energies and rate capabilities than Li-ion batteries. The anode active materials for Na intercalation and the cathode active materials for Na intercalation have lower Na storage capacities as compared with their Li storage capacities. For instance, hard carbon particles are capable of storing Li ions up to 300-360 mAh/g, but the same materials can store Na ions up to 150-250 mAh/g and less than 100 mAh/g for K ion storage.
Instead of hard carbon or other carbonaceous intercalation compound, sodium metal may be used as the anode active material in a sodium metal cell. However, the use of metallic sodium as the anode active material is normally considered undesirable and dangerous due to the dendrite formation, interface aging, and electrolyte incompatibility problems.
Low-capacity anode or cathode active materials are not the only problem associated with the sodium-ion battery or potassium-ion battery. There are serious design and manufacturing issues that the battery industry does not seem to be aware of, or has largely ignored. For instance, despite the seemingly high gravimetric capacities at the electrode level (based on the anode or cathode active material weight alone) as frequently claimed in open literature and patent documents, these electrodes unfortunately fail to provide batteries with high capacities at the battery cell or pack level (based on the total battery cell weight or pack weight). This is due to the notion that, in these reports, the actual active material mass loadings of the electrodes are too low. In most cases, the active material mass loadings of the anode (areal density) is significantly lower than 15 mg/cm2 and mostly <8 mg/cm2 (areal density=the amount of active materials per electrode cross-sectional area along the electrode thickness direction). The cathode active material amount is typically 1.5-2.5 times higher than the anode active material amount in a cell. As a result, the weight proportion of the anode active material (e.g. carbon) in a Na-ion battery cell is typically from 12% to 17%, and that of the cathode active material (e.g. NaxMnO2) from 17% to 35% (mostly <30%). The weight fraction of the cathode and anode active materials combined is typically from 30% to 45% of the cell weight.
The low active material mass loading is primarily due to the inability to obtain thicker electrodes (thicker than 100-200 μm) using the conventional slurry coating procedure. This is not a trivial task as one might think, and in reality the electrode thickness is not a design parameter that can be arbitrarily and freely varied for the purpose of optimizing the cell performance. Contrarily, thicker samples tend to become extremely brittle or of poor structural integrity and would also require the use of large amounts of binder resin. The low areal densities and low volume densities (related to thin electrodes and poor packing density) result in a relatively low volumetric capacity and low volumetric energy density of the battery cells.
With the growing demand for more compact and portable energy storage systems, there is keen interest to increase the utilization of the volume of the batteries. Novel electrode materials and designs that enable high volumetric capacities and high mass loadings are essential to achieving improved cell volumetric capacities and energy densities.
Hence, a general object of the present invention is to provide a rechargeable Na metal cell, K metal cell, hybrid Na/K metal cell, Na-ion cell, K-ion cell, or hybrid Na/K-ion cell that exhibits a high gravimetric energy density, high volumetric energy, high power density, long cycle life, and no danger of explosion due to Na/K metal dendrites. This cell includes the Na or K metal secondary cell, Na-ion cell, K-ion cell, or a non-lithium alkali metal hybrid cell, wherein at least one electrode (the cathode alone or both the anode and cathode) operates on Na or K insertion or intercalation.
One specific technical goal of the present invention is to provide a safe Na- or K-metal based battery having a long cycle life and a gravimetric energy density greater than 150 Wh/Kg and volumetric energy greater than 450 Wh/L, preferably greater than 250 Wh/Kg and 600 Wh/L, and more preferably greater than 300 Wh/Kg and 750 Wh/L (all based on the total cell weight or cell volume).
A specific object of the present invention is to provide a rechargeable non-lithium alkali metal cell based on rational materials and battery designs that overcome or significantly reduce the following issues commonly associated with conventional alkali metal cells: (a) dendrite formation (internal shorting due to sharp dendrite penetrating the separator to reach the cathode); (b) extremely low electric and ionic conductivities of Na intercalation compound in the cathode, requiring large proportion (typically 10-30%) of non-active conductive fillers and having significant proportion of non-accessible or non-reachable cathode active material); and (c) short cycle life. Another object of the present invention is to provide a simple, cost-effective, and easy-to-implement approach to preventing potential Na metal dendrite-induced internal short circuit and thermal runaway problems in various Na metal and Na-ion batteries.